1. Field of the Invention
The invention is directed to a secondary hydraulic steering system for steering a large work vehicle when hydraulic pressure is lost.
2. Description of the Prior Art
Large articulated work vehicles, such as four-wheel drive loaders, four-wheel drive tractors, logging skidders, and other vehicles, use hydraulic systems to articulate and steer the vehicle. Such systems are well known in the art, as disclosed in U.S. Pat. No. 4,109,679. Typically these systems are provided with a hydraulic controller which is coupled to a steering wheel and in response to manipulation of the steering wheel controls the output of pressurized hydraulic fluid to steering cylinders. The hydraulic controller typically comprises a fluid meter and valve structure that are operatively coupled to one another by a mechanical follow-up connection, the fluid meter being formed from a gerotor.
To assure continued steerability of a large work vehicle when hydraulic pressure is lost, these vehicles may be provided with secondary hydraulic pressure systems. One such secondary hydraulic pressure system comprises an electrically powered pump that supplies pressurized hydraulic fluid to the steering circuit when a loss of hydraulic pressure is detected. Another secondary hydraulic pressure steering system comprises hydraulic accumulators for storing hydraulic pressure until needed. With smaller work vehicles, however, secondary hydraulic pressure systems may not be needed Smaller work vehicles can be manually steered by the operator even when hydraulic pressure is lost. This is accomplished because the fluid meter of the controller acts as a pump, supplying hydraulic fluid to the steering cylinders Such a manual system is ineffective with larger vehicles due to the size of the fluid meter.
Gerotors having more than one operational configuration are known in the art as disclosed in U.S. Pat. Nos. 4,480,971 and 4,493,622. More specifically these motors can be operated in a full displacement configuration, or a reduced displacement configuration, depending upon the selected operational configuration. The Eaton Corporation, Hydraulics Division, of Eden Prairie, Minn., is currently marketing a dual displacement motor similar to the one disclosed in the U.S. Pat. No. 4,480,971. This motor is provided with a two-position pilot spool for selecting the operational displacement configuration.